This invention relates to the assembly of electronic circuitry which includes the mounting of discrete electrical components on a circuit substrate, such as a printed circuit board (PCB). These electrical components are decreasing in size in response to the demand for smaller electronic packages, and this in turn requires a corresponding response in the advancement of manufacturing technology.
In the current manufacturing technology, solder is often used to secure an electrical component to the PCB, and also to provide electrical connection between the electrical component and the underlying electrical circuitry on the PCB. Typically, a solder screening process is used to apply the solder to the PCB in preparation for the placement of electrical components. In this process, the layout for the PCB will include solder pads on which electrical components are to be mounted. A stencil on which openings have been created to coincide with the targeted solder pads is placed over the printed circuit board. Solder is deposited on the solder pads by forcing the solder through the stencil and onto the targeted solder pads. The amount of solder deposited will depend of the dimensions of each opening. At this stage, the solder is in the form of a solder paste, which comprises very small solder balls held together with a flux. The solder paste is first heated until the solder is in a molten state, then the solder is solidified by cooling. The term "reflow" is commonly used in the art to describe the transformation of the solder from solid to molten state and back to solid. In some manufacturing processes the electrical components are placed on the solder paste before solder reflow occurs. Thus, the electrical components are secured to the PCB after the solder reflow is completed. However, the solder screening process is sometimes ill-suited for in-line manufacturing. Thus, it is often desirable to completed the solder screening processing off-line by performing solder reflow on the PCB before the electrical components are placed. A second solder reflow is performed after the electrical components are placed.
Several technical problems are encountered with the use of the solder screening process which are exacerbated as the electrical components become smaller. As illustrated in FIG. 2, one of the primary problems in the prior art is the shape of the solder on the solder pad 11 after the solder is reflowed. The reflowed solder 25 on the solder pad 11 sometimes assume a dome-like shape 25, rather than a more desirable planar shape, and as shown in FIG. 3, this can adversely affect the placement of the electrical components 30. The dome-shape in conjunction with the combined tolerances of the placement mechanism and the components, may cause a misalignment of the components on the PCB 50. The ultimate shape of the reflowed solder is influenced by the amount of solder paste deposited on the solder pad. A smaller deposit of solder would lessen of the impact of the shape of the solder deposit on smaller electrical components. However, limitations on the minimum dimensions of stencil openings affect the minimum amount of solder which may be deposited. Furthermore, there is a minimum amount of solder required to ensure proper electrical connection and proper retention of the component to the PCB.
An additional problem with prior art implementations is the resultant increased height of the reflowed solder. This increase in height often varies, which may result in damage to delicate electrical components placed by automatic placement equipment because of an increased in pressure on the components during placement. Thus, a method for reducing solder height while still retaining the minimum amount of solder required was a long-felt but unfulfilled need in the art.
The advent of the increasingly smaller electrical components coupled with the need for more reliability in the manufacturing process for electronic circuitry have elevated the importance of finding solutions to the above-mentioned problems.